A brief analysis of Javascript variable functions_javascript skills
There are two types of values that can be stored in JavaScript variables: original values and reference values.
The original value is stored in a simple field on the stack, that is, the value is stored directly in the location marked by the variable.
The reference value is stored in the object in the heap, and the stack variable stores the pointer value pointing to the object in the heap.
There are 5 basic types in javascript: Undefined, Null, Boolean, Number, String.
Reference types are actually objects, similar to the concept of class instances in other languages.
var b = true; // Store on the stack
var num = 20; //Stored on the stack
var b = new Boolen(true); //Stored in the heap
var num = new Number(20); //Stored in the heap
The usual way to generate objects:
new type name var obj = new object()
If there are no parameters, it can also be written as var obj = new object;
You can also use object literals Quantity generation object var obj = {}
2. Function
In JavaScript, functions are objects. They should be treated like other objects in JavaScript. Each function has two implicitly attached Additional parameters this, arguments.
Functions can be: assigned to variables, used as attributes of other objects, used as parameters of other functions, used as return values, and functions can also be created using literals.
Function context:
In object-oriented languages, use the this keyword to refer to the current instance of a class object. The this keyword in JavaScript is different from this in object-oriented languages. In JavaScript, a function is an object, and this refers to the function context in which the current function is called.
You can explicitly specify function context through the call() and apply() methods. The first parameter of Call is used as the context of the calling function, and the other parameters are passed into the called function as parameters of the called function. Apply() is similar to Call(), except that the second parameter is an array.
var obj = {
m:"hello"
}
var m="hi";
var say=function()
{
alert(this.m); //this points to the function calling context
}
say();//hi, window is the calling context
say.call(obj);// hello, obj is the function calling context at this time
say.call(window);// hi, window is Calling context
Scope:
Parameters and variables defined in a function are not visible outside the function, and variables defined anywhere in a function are not visible anywhere in the function. Everywhere is visible.
var obj = function() {
var num = 1;
return { getValue: function() {
alert(num); //undefined
var num = 2;
alert(num); //2
}
}
} ();
obj.getValue();
Closure
The so-called closure means that the function can use variables defined outside the function, and the function can access the variables when it is created. context.
var hello = "hello word!";
function sayHello() {
alert(hello);
}
sayHello();
var obj = function() {
var value = 0;
return {
setValue: function(val) {
value = typeof val === 'number' ? val : 1;
},
getValue: function() {
return value;
}
}
} ();
obj.setValue('a');
alert(obj.getValue()); //1
Pay attention to the last line (), () is the call operator, which means that the function is called immediately and returns the call result. So obj does not refer to a function, but to an object returned by the function that contains two methods, and these two methods enjoy the privilege of accessing the value variable.
Let’s take another example of an internal function accessing local variables of an external function that is widely circulated on the Internet to illustrate closure. Click the corresponding list item to pop up the corresponding sequence number.
var test = function() {
var num = document.getElementsByTagName("li");
var i;
for (i = 0; i < num.length; i ) {
num[i].onclick = function() {
alert(i); //Internal functions can access external function variables, the final value of i is 3,
/ /Instead of the i value in the constructor, so 3
}
}
alert(i); //3
}
test();
var num = document.getElementsByTagName("li");
var i;
for (i = 0; i < num.length; i ) {
num[i].onclick = function(i) {
return function() {
alert(i 1);
}
} (i); //Every time the constructor Immediately pass the i value in for execution. Now the function bound to onclick is the function returned after executing the
//line
}
alert(i); //3
}
test( );
Hot AI Tools
Undresser.AI Undress
AI-powered app for creating realistic nude photos
AI Clothes Remover
Online AI tool for removing clothes from photos.
Undress AI Tool
Undress images for free
Clothoff.io
AI clothes remover
Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!
Hot Article
Hot Tools
Notepad++7.3.1
Easy-to-use and free code editor
SublimeText3 Chinese version
Chinese version, very easy to use
Zend Studio 13.0.1
Powerful PHP integrated development environment
Dreamweaver CS6
Visual web development tools
SublimeText3 Mac version
God-level code editing software (SublimeText3)
Hot Topics
1666
14
1426
52
1328
25
1273
29
1255
24
JavaScript Engines: Comparing Implementations
Apr 13, 2025 am 12:05 AM
Different JavaScript engines have different effects when parsing and executing JavaScript code, because the implementation principles and optimization strategies of each engine differ. 1. Lexical analysis: convert source code into lexical unit. 2. Grammar analysis: Generate an abstract syntax tree. 3. Optimization and compilation: Generate machine code through the JIT compiler. 4. Execute: Run the machine code. V8 engine optimizes through instant compilation and hidden class, SpiderMonkey uses a type inference system, resulting in different performance performance on the same code.
Python vs. JavaScript: The Learning Curve and Ease of Use
Apr 16, 2025 am 12:12 AM
Python is more suitable for beginners, with a smooth learning curve and concise syntax; JavaScript is suitable for front-end development, with a steep learning curve and flexible syntax. 1. Python syntax is intuitive and suitable for data science and back-end development. 2. JavaScript is flexible and widely used in front-end and server-side programming.
From C/C to JavaScript: How It All Works
Apr 14, 2025 am 12:05 AM
The shift from C/C to JavaScript requires adapting to dynamic typing, garbage collection and asynchronous programming. 1) C/C is a statically typed language that requires manual memory management, while JavaScript is dynamically typed and garbage collection is automatically processed. 2) C/C needs to be compiled into machine code, while JavaScript is an interpreted language. 3) JavaScript introduces concepts such as closures, prototype chains and Promise, which enhances flexibility and asynchronous programming capabilities.
JavaScript and the Web: Core Functionality and Use Cases
Apr 18, 2025 am 12:19 AM
The main uses of JavaScript in web development include client interaction, form verification and asynchronous communication. 1) Dynamic content update and user interaction through DOM operations; 2) Client verification is carried out before the user submits data to improve the user experience; 3) Refreshless communication with the server is achieved through AJAX technology.
JavaScript in Action: Real-World Examples and Projects
Apr 19, 2025 am 12:13 AM
JavaScript's application in the real world includes front-end and back-end development. 1) Display front-end applications by building a TODO list application, involving DOM operations and event processing. 2) Build RESTfulAPI through Node.js and Express to demonstrate back-end applications.
Understanding the JavaScript Engine: Implementation Details
Apr 17, 2025 am 12:05 AM
Understanding how JavaScript engine works internally is important to developers because it helps write more efficient code and understand performance bottlenecks and optimization strategies. 1) The engine's workflow includes three stages: parsing, compiling and execution; 2) During the execution process, the engine will perform dynamic optimization, such as inline cache and hidden classes; 3) Best practices include avoiding global variables, optimizing loops, using const and lets, and avoiding excessive use of closures.
Python vs. JavaScript: Community, Libraries, and Resources
Apr 15, 2025 am 12:16 AM
Python and JavaScript have their own advantages and disadvantages in terms of community, libraries and resources. 1) The Python community is friendly and suitable for beginners, but the front-end development resources are not as rich as JavaScript. 2) Python is powerful in data science and machine learning libraries, while JavaScript is better in front-end development libraries and frameworks. 3) Both have rich learning resources, but Python is suitable for starting with official documents, while JavaScript is better with MDNWebDocs. The choice should be based on project needs and personal interests.
Python vs. JavaScript: Development Environments and Tools
Apr 26, 2025 am 12:09 AM
Both Python and JavaScript's choices in development environments are important. 1) Python's development environment includes PyCharm, JupyterNotebook and Anaconda, which are suitable for data science and rapid prototyping. 2) The development environment of JavaScript includes Node.js, VSCode and Webpack, which are suitable for front-end and back-end development. Choosing the right tools according to project needs can improve development efficiency and project success rate.
